1. Field of Invention
This invention relates to an exhaust system for adsorbing contaminants from an exhaust stream of a molten carbonate fuel cell. More particularly, it relates to a filter system for adsorbing contaminants from a molten carbonate fuel cell, wherein the filter system includes a filter substrate, an inorganic adsorbent secured to the filter substrate by an inorganic binder and an acidic material coated onto the filter substrate. The invention also relates to a process for the preparation of and use of the filter system within a molten carbonate fuel cell system.
2. Description of Related Art
Fuel cell systems represent a new technological approach to supplying energy. There has been a great interest in fuel cell technology because energy can be produced by such cells with high efficiency without creating many of the problems that are common when using fossil fuels, especially the environmental problems.
A fuel cell is an electrochemical device which continuously converts chemical energy of a reducing agent and an oxidant fuel to electrical energy by a process involving an essentially invariant electrode—an electrolyte system. Fuel cells work at high efficiency with an emission level for undesirable pollutants far below that of the strictest industrial standards for conventional energy sources.
There are a number of different types of fuel cells with some classified based on working temperature and others classified by the fuels or oxidants that are used. Another method for distinguishing among different types of fuel cells is by the type of electrolyte that is used, such as alkaline fuel cells, phosphoric acid fuel cells, solid oxide fuel cells, proton exchange membrane fuel cells and molten carbonate fuel cells.
Molten carbonate fuel cells (MCFC) operate at about 600° C.-650° C. and typically comprise a reformer for converting hydrocarbon fuels to hydrogen and carbon monoxide, a burner and a plurality of molten carbonate fuel cell stacks. These fuel cells operate by reacting oxygen contained in an oxidant stream with carbon dioxide and free electrons at a cathode to produce carbonate ions. These carbonate ions migrate across a molten carbonate electrolyte to an anode where they react with hydrogen and carbon monoxide contained in a fuel system to produce water, carbon dioxide and free electrons. The free electrons pass through an external load back to the cathode thereby producing electricity while the carbon dioxide, water and any remaining hydrogen exits the anode into an exhaust stream. Typical MCFC systems also include an anode exhaust, downstream oxidation catalyst, which is located downstream from the fuel cell electrodes, and which oxidizes hydrocarbons, hydrogen and carbon monoxide contained in the exhaust stream. A general discussion of fuel cells, including molten carbonate fuel cell systems, is disclosed in Kordesch, et al., Environment Impact of Fuel Cell Technology (Chem. Rev. 1995, pp. 191-207). See also U.S. Pat. Nos. 5,308,456, 5,213,912 and 5,079,103.
The MCFC uses a molten carbonate salt mixture as the electrolyte. The composition of the electrolyte may vary but usually includes lithium carbonate and potassium carbonate. Because the operating temperature of the MCFC is about 600° C.-650°C., the carbonate salt mixture is liquid. During operation, some alkali carbonates are often discharged from the fuel cell stacks into the exhaust stream. Such alkali carbonates are poisonous to the downstream oxidation catalyst. These alkali carbonates may also poison other components of the fuel cell system. Thus, it is imperative that these alkali carbonate contaminants be removed from the exhaust stream of the MCFC system.
Monolithic catalyst adsorbents are commonly utilized to remove contaminants from various types of exhaust streams. For example, monolithic catalyst products having a cellular or honeycomb structure have been utilized in stationary emission control situations, food cooking facilities as disclosed in U.S. Pat. No. 5,431,887 and for chemical synthesis and processing facilities. Monolithic catalyst products are also utilized in the automotive industry and for NOx conversion in feed streams.
Catalyst adsorbent products of this type are generally fabricated by preparing a substantially homogeneous cellular ceramic or metallic monolithic structure and securing a catalyst adsorbent material to that monolithic structure.
The catalyst adsorbent product can also be formed by impregnating the monolithic structure with the catalyst adsorbent material and then heat treating the coated monolith to produce the final product.
Conventional washcoating techniques for production of catalytic monolith products generally comprise preparing a coating formed from a high surface area oxide blended with one or more catalytic adsorbents and dipping the monolith structure into that coating blend. These washcoats are then secured to the monolithic structure. Methods of coating a preformed high surface area washcoat onto a monolithic support are disclosed in U.S. Pat. Nos. 4,900,712, 5,431,887, 5,556,189 and 5,693,298.
Several patents disclose the combination of catalytically active washcoat compositions comprising an alumina binder and catalytically active noble metals for the oxidation of hydrocarbons and/or the reduction of nitrogen oxides for field abatement, automotive exhaust control and the like, including, for example, U.S. Pat. Nos. 6,150,291, 6,093,378, 6,080,377, 5,773,423 and 5,354,720.
In U.S. Pat. No. 5,206,202 a honeycomb substrate structure is coated with a catalytic iron/vanadium mixture blended with an alumina or titania binder.
In another example, U.S. Pat. No. 5,776,423 discloses a process for the preparation of a metallic zeolite catalyst adsorbent for NOx abatement. This patent discloses a process for the formation of a copper and iron ZSM-5 zeolite product using an alumina coating and silica sol binder. See also U.S. Pat. No. 6,150,291 and 5,354,720.
Other catalyst adsorbents for purifying exhaust gases are disclosed by U.S. Pat. Nos. 5,443,803 and 5,354,720. Each of these patents discloses the use of a silica sol as a binder for an active metal coated onto a monolithic structure.
U.S. Pat. No. 6,004,896 discloses a hydrocarbon absorber and a method of making same. A ZSM-5 zeolite is treated with phosphoric acid and then blended into a slurry for washcoating onto a monolithic structure. The preferred binder for the slurry is alumina, although the composition of the binder may also include silica and/or zirconia or their precursors.
U.S. Pat. No. 5,813,764 discloses a catalytic microcalorimeter sensor for monitoring exhaust gas conversions. The sensor disclosed is a washcoat obtained by mixing silica sol and alumina sol. The washcoat is used as the catalytic material itself and the components thereof are not used as binder materials. This patent also discloses the use of catalytically active metal particles. See the related patent, U.S. Pat. No. 5,707,148.
While monolithic catalyst adsorbent products having a cellular or honeycomb structure have increasingly been utilized for emission control situations, such as in the automotive industry or for the adsorption of gases in feed and exhaust streams, such as are created during some food preparation processes, the use of a catalyst adsorbent for adsorbing inorganic contaminants in a fuel cell, particularly a molten carbonate fuel cell, has not been considered.
Accordingly, it is an object of this invention to disclose an exhaust treatment system for adsorbing contaminants from a molten carbonate fuel cell (MCFC) system including a filter system and an oxidation catalyst.
It is a further object of the invention to disclose a filter system for filtering an exhaust stream of a MCFC which absorbs alkali carbonate contaminants, which system comprises a filter substrate, an inorganic adsorbent secured to the filter substrate by an inorganic binder and an acidic material.
It is a still further object of the invention to disclose a filter system contained in the exhaust treatment system for adsorbing specific contaminants from an exhaust stream of a MCFC prior to passage of the exhaust stream through an oxidation catalyst comprising a filter substrate, a high surface area inorganic adsorbent secured to the filter substrate by an inorganic binder and an inorganic acid secured to the filter substrate.
It is a still further object of the invention to disclose a process for filtering contaminants which are present in an exhaust stream of a MCFC by passing the exhaust fuel stream through an exhaust treatment system.
It is a still further object of the invention to disclose a process for filtering contaminants which are present in the exhaust stream of a MCFC comprising passing a fuel stream through the MCFC and filtering contaminants from the exhaust stream by use of a filter system of an exhaust treatment system prior to passing the filtered exhaust stream through an oxidation catalyst system.
It is a still further object of the invention to disclose a process for preparing a filter system of an exhaust treatment system for filtering exhaust gases from a MCFC comprising preparing a filtering substrate, coating the filtering substrate with an inorganic adsorbent by use of an inorganic binder and treating the coated filter substrate with an acidic material.
It is a further object of the invention to disclose a process for preparing a filter system for filtering exhaust gases from a MCFC prior to passage of the exhaust gases through an oxidation catalyst comprising preparing a filter substrate and coating the filter substrate in a single coating step with a solution comprising an inorganic adsorbent, an inorganic acidic material and an inorganic binder.
These and other aspects of the invention are obtained by various designs of the filter system, the process of its manufacture and the process of its use.